What defines performance in a 100-km ultramarathon? electrocardiographic and body composition predictors in middle-aged and senior amateur runners

Background Ultramarathon running is characterized by prolonged physiological stress and cumulative fatigue. During 100-km races, performance depends on maintaining physiological balance under sustained load and requires effective cardiovascular and metabolic adaptation. Both baseline body composition and electrocardiographic characteristics, as well as their exercise-induced changes, may contribute to differences in race time. Purpose To identify baseline body composition and electrocardiographic predictors of 100-km ultramarathon race time in middle-aged and senior amateur runners, and to determine whether race-induced physiological changes independently relate to performance beyond training load. Methods Resting 12-lead electrocardiography and body composition analysis were performed before and immediately after a 100-km ultramarathon in 42 amateur ultramarathon runners (mean age 52.4 years; 44–70 years; 24 men). Weekly training volume was obtained by structured interview. Associations with race time were analysed using Pearson correlation and multivariable linear regression adjusted for age, sex, and training volume. Results The 100-km race significantly reduced body weight (-2.02 kg, p<0.001), body fat percentage (-1.57%, p<0.001), and muscle mass (-0.64 kg, p=0.012). Average resting heart rate increased (+22.8 bpm, p<0.001), accompanied by prolongation of P-wave duration (+6.39 ms, p<0.001) and increased P-wave amplitude (+0.46 mm, p<0.001). Shorter race time correlated with higher weekly training volume (r=-0.38, p<0.05). Among baseline variables, body fat percentage showed a strong positive correlation with race time (r=0.56, p<0.001) and muscle mass was not associated with performance. Baseline electrocardiographic parameters did not correlate with race time. Greater post-race increase in heart rate correlated with shorter race time (r =-0.55, p<0.001). A larger increase in P-wave amplitude showed a modest association with shorter race time (r=-0.35, p=0.029). In multivariable analysis, baseline body fat percentage remained positively associated with race time (β > 0; additional explained variance 31%, p<0.001). Among electrocardiographic characteristics, greater post-race increase in heart rate was related to shorter race time (β < 0; additional explained variance 19%, p=0.014), whereas longer post-race P-wave duration was associated with longer race time (β > 0; additional explained variance 20%, p=0.011). Conclusions Beyond training load, 100-km ultramarathon performance reflects both baseline body composition and the magnitude and direction of acute cardiac response to extreme endurance stress. Lower baseline body fat, a greater post-race heart rate increase, and less pronounced post-race P-wave prolongation independently characterize faster race performance.